Ubuntu 16.04 LTS 上的 c++ 编译问题？

Question

所以我有一个调试器问题。 所以我创建了一个 c++ 程序并在 Ubuntu 16.04 上运行它。 但是，当尝试通过在命令行中键入g++ decrypt.cpp caesor\ cipher.cpp -o decrypt进行编译时，它给了我一些错误。 这是我很长时间以来第一次在 c++ 上工作，希望你们能发现一些我看不到的东西。 这是下面的代码和错误。 谢谢你

======================================

解密.cpp

#include"decrypt.h"
#include<fstream>
#include<string>
using namespace std;
#include<iostream>
Decrypt::Decrypt(string text)
{
    str = text;
    zero_analysis_array();
    //E A R I O are top 5 most commonly used letters in english words
     //initializing keys arrays with thier position in alphabets
    keys[0] = 'E' - 'A';
    keys[1] = 'A' - 'A';
    keys[2] = 'R' - 'A';
    keys[3] = 'I' - 'A';
    keys[4] = 'O' - 'A';
}
void Decrypt::zero_analysis_array()
{
    for (int i = 0; i < 26; i++)
    {
        freq[i] = 0;
    }

}

void Decrypt::character_count()
{
    char ch;
    for (int i = 0; i < str.size(); i++)
    {
        if (is_alpha(str[i]))
        {
            ch = str[i];
            if (!is_upper(ch))
                ch = to_upper(str[i]);
            int a = ch - 'A';
            if (a > 26 || a < 0)
                cout << "a";
            freq[ch - 'A']++;
        }
    }
}

void Decrypt::print_analysis_array()
{
    char alpha = 'A';
    cout << "\n\n";
    for (int i = 0; i < 26; i++)
    {
        cout << char(alpha + i) << " occurs " << freq[i] << " times\n";
    }
    int ind = max_index(), key;
    cout << "Possible keys:\n";
    for (int i = 0; i < 5; i++)
    {
        key = ind - keys[i];
        if (key < 0)
            key = key + 26;
        cout << key << "\n";
    }

}
string Decrypt::decrypt(int key)
{
    string decrypt = str;
    int code, a, b;
    char ch;
    for (int i = 0; i < decrypt.size(); i++)
    {
        if (is_alpha(decrypt[i]))
        {
            ch = 'A';
            if (is_upper(decrypt[i]) == false)
                ch = 'a';
            a = (decrypt[i] - ch);
            b = a + 26 - key;
            code = b % 26;
            decrypt[i] = ch + code;
        }
    }
    return decrypt;

}
void Decrypt::write_to_file(string fname, string text)
{
    fstream file;
    file.open(fname, ios::out);
    file << text;
    file.close();
}
void Decrypt::decryption(string fname)
{
    character_count();
    print_analysis_array();
    string decrypted;
    int key;
    char choice;
    do
    {
        cout << "Select/Enter key(0-26):";
        cin >> key;
        while (key < 0 || key>26)
        {
            cout << "Invalid Key!! Must be between 0-26\nEnter Again:";
            cin >> key;
        }
        decrypted = decrypt(key);
        cout << "\nHere is decrypted text:\n\n" << decrypted;
        cout << "\n\nDo you want to try again with different key? (y/n):";
        cin >> choice;
        while (choice != 'Y' && choice != 'y' && choice != 'n' && choice != 'N')
        {
            cout << "Invalid choice!!.\nEnter (y/n):";
            cin >> choice;
        }

    } while (choice == 'y' || choice == 'Y');
    write_to_file(fname, decrypted);
    cout << "Decrypted text written to file " << fname << "\n";
}
int Decrypt::max_index()
{
    int maxIndex = 0;
    int maxFreq = freq[0];
    for (int i = 0; i < 26; i++)
    {
        if (maxFreq < freq[i])
        {
            maxIndex = i;
            maxFreq = freq[i];
        }
    }
    return maxIndex;
}
char Decrypt::to_upper(char ch)
{
    if (!is_upper(ch))
        return ch - 32;
    return ch;
}
bool Decrypt::is_alpha(char ch)
{
    return((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z'));
}
bool Decrypt::is_upper(char ch)
{
    return (ch >= 'A' && ch <= 'Z');
}

==================================

凯撒密码.cpp

#include<fstream>
#include <iostream>
#include"decrypt.h"
int main(int argc,char*argv[])
{
    if (argc < 2)
    {
        cout << "Invalid no of Arguments!!!\n";
        exit(0);
    }
    string text=read_from_file(argv[1]);
    cout << text;
    Decrypt d(text);
    d.decryption(argv[2]);
}

string read_from_file(char* fname)
{
    fstream file;
    string line,text="";
    file.open(fname, ios::in);
    while (getline(file, line))
    {
        text = text + line;
    }
    return text;

}

==========================

decrypt.cpp: In member function ‘void Decrypt::write_to_file(std::__cxx11::string, std::__cxx11::string)’:
decrypt.cpp:88:30: error: no matching function for call to ‘std::basic_fstream<char>::open(std::__cxx11::string&, const openmode&)’
     file.open(fname, ios::out);
                              ^
In file included from decrypt.cpp:2:0:
/usr/include/c++/5/fstream:1001:7: note: candidate: void std::basic_fstream<_CharT, _Traits>::open(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits<char>; std::ios_base::openmode = std::_Ios_Openmode]
       open(const char* __s,
       ^
/usr/include/c++/5/fstream:1001:7: note:   no known conversion for argument 1 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘const char*’
caesor cipher.cpp: In function ‘int main(int, char**)’:
caesor cipher.cpp:9:15: error: ‘exit’ was not declared in this scope
         exit(0);

Answer 1

好吧，编译器实际上会告诉您程序有什么问题（并且还会提示错误所在的位置）。

decrypt.cpp:88:30: error: no matching function for call to ‘std::basic_fstream<char>::open(std::__cxx11::string&, const openmode&)’
     file.open(fname, ios::out);
                              ^

这意味着您对std::fstream::open的调用不正确。 它需要一个 C 字符串，而不是你给它的 C++ std::string 。 这里最简单的解决方法是将调用更改为open ，如下所示：

file.open(fname.c_str(), ios::out);

第二个错误说它找不到名为exit的 function 。 这是因为编译器还不知道这个 function。 您需要在声明 function 的地方包含适当的 header （在本例中为stdlib.h ），以便编译器知道存在一个名为exit的 ZC1C425268E68385D1AB5074C17A94F1。

Answer 2

你的错误是不言自明的。 它说：

error: no matching function for call to ‘std::basic_fstream<char>::open(std::__cxx11::string&, const openmode&)’

但它让您知道最接近的匹配是：

note: candidate: void std::basic_fstream<_CharT, _Traits>::open(const char*, std::ios_base::openmode)

如果您注意到，最大的区别在于string与char * 。 所以给它它正在寻找的 C 字符串：

file.open(fname.c_str(), ios::in);

但是，从 C++11 开始， open()应该接受std::string 。 因此，假设您使用的是g++ ，您也可以添加

-std=c++11

编译时。